1. Field of the Invention
The present invention broadly relates to the fields of medical imaging, diagnostics, and pharmaceutical therapy.
2. Description of the Related Art
Radiopharmaceuticals
In general, radiopharmaceuticals provide vital information that aids in the diagnosis and therapy of a variety of medical diseases (Hom and Katzenellenbogen, 1997). Radiopharmaceuticals relay data on tissue shape, biochemical function, and localization within the body by use of radionuclides which act as imaging agents. Radionuclides include free chemical species, such as the gas 133Xe, or the ions 123I- and 201T1-, which are covalently or coordinately bound to a larger organic or inorganic chemical moiety. Images are generated from the signal arising from radioactive decay of the nuclide which is distributed in tissues according to the properties of the larger moiety to which the radionuclide is bound. Radionuclides most commonly used for medical imaging include 11C (t1/2=20.4 min), 13N (t1/2=9.97 min), 15O (t1/2=2.03 min), 18F (t1/2=109.7 min), 64Cu (t1/2=12 h), 68Ga (t1/2=68 min), and 94mTc (t1/2=53 min) for positron emission tomography (PET) and 67Ga (t1/2=68 min), 99mTc (t1/2=6 h), 123I (t1/2=13 h) and 201T1 (t1/2=73.5 h) for single photon emission computed tomography (SPECT) (Hom and Katzenellenbogen, 1997).
SPECT and PET imaging provide accurate data on radionuclide distribution in the desired target tissue by detection of the gamma photons that result from radionuclide decay. The high degree of spatial resolution of modern commercial SPECT and PET scanners enables images to be generated that map the radionuclide decay events into an image that reflects the distribution of the agent in the body. These images thus contain anatomic and functional information useful in medical diagnosis. Radionuclide decay can also be expolited for therapeutic effect. When radionuclides decay in such a manner as to deposit radiation energy in or near target cells or tissues, therapeutically relevant doses of radioactivity are deposited within the tissues.
The tissue specificity or targeting properties of radiopharmaceuticals often depends largely on overall size, charge, or physical state (Hom and Katzenellenbogen, 1997). Certain radiopharmaceuticals have been synthesized that demonstrate specific binding to, for example, a specific hormone, neurotransmitter, cell surface or drug receptor, enzyme, or high affinity transport systems. When select receptors, enzymes and the like are known to be involved in the regulation of a wide variety of vital bodily functions, targeted imaging agents such as those combining a chemical constituent having specific binding properties with a radionuclide are especially useful in the diagnosis or staging of a variety of disease states. For example, diseases in which such receptors are functioning abnormally or are distributed in an abnormal fashion are especially amenable to diagnosis using such radiopharmaceuticals. The success of therapy of such diseases can also be monitored using radiopharmaceuticals (Hom and Katzenellenbogen, 1997).
Recent advances in molecular, structural and computational biology have begun to provide insights into the structure of molecular targets, receptors and enzymes and these insights can be used to design various targeting molecules, or ligands. The localization of molecular targets within tissues also directly impacts the development of new radiopharmaceuticals. Most importantly, the location of a receptor or enzyme activity in the body (i.e., peripheral sites versus brain sites), and the receptor's subcellular location (i.e., on the cell surface versus intracellular) determines whether a radiopharmaceutical injected intravenously will need to traverse one or more membrane and cellular barriers to reach the target. Moreover, the structure of the molecular target, its localization in tissues, and the nature of the target's interaction with its natural ligand are all factors that help determine the degree to which large ligands or ligands with large substituents may be tolerated (Han et al., 1996). For example, radiopharmaceuticals that target cell surface receptors encounter no membrane barriers to reach their target. Thus, natural ligands for these targets can be relatively large, and are often charged and consequently large radiopharmaceutical molecules can be used for such targets. Conversely, a radiopharmaceutical which must reach a target within the central nervous system must traverse the blood-brain barrier formed by endothelial cells of the brain. Thus, design of radiopharmaceuticals for targets within the central nervous system favors minimal size and molecular weight (Dishino, 1983; Eckelman, 1995; Hom and Katzenellenbogen, 1997; Papadopoulos et al., 1993).
A focus of recent research has been the development of radiopharmaceuticals targeting cell surface receptors whose natural ligands are peptides. Peptide-based radiopharmaceuticals include a derivatizing group or chelating structure coupled to a peptide, with a radionuclide held by the chelating structure. Peptide-based imaging agents have been described (Lister-James et al., 1997a; Lister-James et al., 1997b; Polyakov et al., 2000), especially those that incorporate technetium-99m (Tc-99m) as the radionuclide, the most commonly used isotope in medical imaging. A variety of metal chelation systems have been developed for synthesis of radioisotopic and magnetic resonance peptide-based imaging agents. Peptide-based agents conventionally target extracellular or externally oriented membrane bound receptors (Hom and Katzenellenbogen, 1997) because the charge, relatively large size, and pharmacokinetic properties of typical peptide structures do not allow diffusion across the lipid bilayer of the cell plasma membrane of cells. For smaller peptides, the size of the added derivatizing group or chelating structure for carrying the radionuclide substantially impacts the in vitro binding and in vivo distribution properties of these compounds (Babich, 1995; Liu et al., 1996). Thus, the design of peptide metal chelates which can report on the functional status or biological activity of targets in the central nervous system is a significant challenge. Until now, peptide-based imaging agents that successfully target receptors or biological activities within the central nervous system have not been described. Attempts to design Tc-99m labeled chrysamine G (CG) and Congo Red (CR) derivatives or mixed functionalities such as isonitriles have been unsuccessful (Dezutter et al., 1999a; Dezutter et al., 1999b; Han et al., 1996). Despite having neutral [TcvO]3+N2S2 cores, high conjugation, and high binding affinity, these agents are unable to permeate the intact blood-brain barrier.
Alzheimer's Disease
Recent estimates indicate that approximately 4 million Americans suffer from Alzheimer's disease (AD), a progressive neurodegenerative disorder with an estimated annual healthcare cost of $100 billion (Schumock, 1998). The clinical symptoms of AD include cognitive decline, irreversible loss of memory, disorientation, and language impairment (McKhann, 1984).
The AD brain is associated with loss of neurons in regions of the brain responsible for learning and memory (e.g., hippocampus) and involve the appearance of two distinct abnormal proteinaceous deposits: extracellular amyloid plaques, that are characteristic of AD, and intracellular neurofibrillary tangles (NFTs) that are found in other neurodegenerative disorders (McKhann, 1984; Weiner, 1997; Yanker, 1996). Amyloid plaques consist of dystrophic neurites, altered astrocytes, and microglia surrounding an insoluble fibrillar core comprised of amyloid β-proteins (Aβ). The family of amyloid β-proteins includes predominantly two variants: Aβ 40, which contains 40 amino acids, and Aβ 42 which is a form believed to be relatively more dangerous and which consists of 42 amino acids (Lansbury, 1996). Aβ is known to be derived from the ubiquitously expressed cell surface amyloid precursor protein (APP) (Games et. al., 1995; Hsiao et al., 1996; Teller, 1996).
Several lines of investigation suggest that overexpression of Aβ is an initiating event in the AD pathogenic cascade. Such evidence includes: a) overexpression of amyloid precursor protein (APP; a transmembrane protein encoded on chromosome 21) is characteristic of Down's Syndrome (DS) and early onset AD has been shown to be a virtual certainty in these patients (Lernere et. al., 1996b; Teller, 1996); b) missense mutations in APP are known as likely early triggers of AD; c) mutations in the presenilin proteins that may have a role in early onset AD have been shown to increase the expression of variant Aβ 42 (Lemere et. al., 1996a; Scheuner et. al., 1996; Selkoe, 1997); and d) transgenic mice that overexpress APP have been shown to develop AD-like neuropathology (Games et. al., 1995; Hsiao et al., 1996).
Currently, AD is diagnosed based on direct clinical observation of cognitive decline, coupled with the systematic elimination of other possible causes of those symptoms (McKhann, 1984; Weiner, 1997). No definitive premortem diagnostic procedure exists for AD, and while clinical observations suggest that amyloid formation precedes neurodegeneration, postmortem neuropathological examinations of amyloid plaques and neurofibrillary tangles (NFTs) typically provide the only direct evidence of the disease. Although the quantity of fibrillar amyloid roughly correlates with severity of symptoms at the time of death, the temporal relationship between amyloid deposition, neuronal loss, and cognitive decline is unclear.
Non-invasive AD Diagnostics
Certain non-invasive AD diagnostic probes are known, and hold some promise for enabling in vivo evaluation of the presence and/or extent of brain amyloid. Known non-invasive AD diagnostic probes include: a) Congo Red derivatized small organic molecules (Dezutter et al., 1999b; Klunk et al., 2002; Skovronsky et al., 2000); b) anti-Aβ monoclonal antibodies that bind specific amino acid residues of Aβ1-42/43 (Majocha et al., 1992; Walker et al., 1994); c) Aβ1-40 peptide derivatized with putrescine for increased permeability across the BBB, with appended chelation cores holding gadolinium (Gd-DTPA) or monocrystalline iron oxide nanoparticles (MION) (see, e.g. Weissleder et al., 2000); and d) iodine-123/125 and carbon-11 labeled thioflavin-based organic compounds that have been developed for in vivo labeling of Aβ plaques (Klunk et al., 2001; Kung et al., 2002).
However, these known imaging agents bear significant limitations. The Congo Red derivatized compounds are neutral, small molecular weight compounds which can permeate the blood-brain barrier, and can provide localization of an Aβ-targeted probe, but do not provide quantification capabilities. Anti-Aβ monoclonal antibodies do not readily permeate the blood-brain barrier. Aβ1-40 derivatized peptides bind or associate with plaques, and carry promise for detection of plaques through MRI, but such molecules do not readily permeate the blood brain barrier and require assistance from mannitol administration to induce permeation (Wadghiri et al., 2003). Such a procedure is unlikely to be approved for routine diagnostic use. Putrescine derivatized Aβ shows some permeability (Poduslo et al., 2002), but labeling with iodine-125 is susceptible to metabolism through deiodination reactions. Further, Aβ itself is known to be toxic (Kowall et al., 1992). Still further, studies using 3H-Aβ 1-40 with RBE4 cell monolayers (as a model of BBB permeability) in transwell experiments also indicate that Aβ1-40 is not transported across the monolayer. Finally, iodine-123/125 and carbon-11 labeled thioflavin-based organic compounds are promising, but agents labeled with iodine are prone to de-iodination reactions due to lability of the carbon-iodine bond when exposed to stringent in vivo environments over time. Carbon-11 agents hold some promise, but their extremely short half-life (20.4 minutes) restricts their accessibility to serve as efficient screening tools to those clinics associated with cyclotrons.
Thus, known non-invasive imaging tools as they apply to in vivo diagnosis of AD are currently quite limited for a variety of reasons. A clear need remains for tools and methods enabling premortem diagnosis of AD, elucidation of the pathogenesis of AD, and efficient monitoring of patients undergoing anti-amyloid therapeutic treatment. In particular, a need exists for non-invasive imaging techniques for visualizing AD-associated changes in the brain.